The Schulz U.S. Pat. No. 4,642,863 describes a process for making a hollow article, especially a hollow airfoil, from multiple sections by superplastic forming and diffusion bonding (SPF/DB). In particular, the method employs an internal mandrel having a non-reactive surface by virtue of being coated with a release agent, such a yttria or boron nitride. One or more mandrels are sandwiched between the metallic sheets to allow contact therebetween at only desired locations; for example, at the leading edge, trailing edge and an intermediate rib to be formed between the sheets intermediate the leading and trailing edge. The mandrels provide control of the internal radius of the leading and trailing edges. The assembly of the metallic sheets and mandrels is maintained under coordinated temperature-gas pressure-time conditions to diffusion bond the sheets together at those locations in contact by virtue of the configuration of the mandrel. After diffusion bonding, an inert gas is injected between the sections while the bonded assembly is held in a die set of desired configuration and heated to a superplastic forming temperature. The gas pressure is controlled to superplastically inflate the bonded assembly to conform to the die shape.
It is an object of the invention to provide a method of making a hollow article, such as a hollow airfoil, where sequential low and high pressure diffusion bonding conditions are selected in conjunction with alternate external die support of the sections to avoid distortion of one section altogether while controllably limiting distortion of the other section.
It is another object of the invention to provide a method of making a hollow article, such as a hollow airfoil, from preformed sections where a fluid pressure inflation step is conducted subsequent to or preferably concurrently with high pressure diffusion bonding to remove any minor distortion of the distorted section.
It is another object of the invention to provide a method of making a hollow article, such as a hollow airfoil, where there is no need for superplastic forming of the sections to impart a desired configuration thereto or for internal mandrels between the sections.